1. Field of the Invention
This invention relates to electronic circuits, and more particularly, to efficiently testing circuits with hysteresis.
2. Description of the Relevant Art
The output of a system with hysteresis depends both on a current input value and a current output value, which is based on a past input value. This system has more than one stable output state. The system may snap, or quickly transition, from one stable state to another stable state in response to an input trigger pulse and its history. A circuit with hysteresis may have at least two stable states. The circuit with hysteresis may have at least two trigger points, such as a high trigger point and a low trigger point.
In response to an input signal being above a high trigger point after the input had previously been below the low trigger point, the output of the circuit with hysteresis changes from its current stable value to another stable value. In response to the input signal being below a low trigger point after the input had previously been above the high trigger point, the output changes from its current value to another stable value. The current stable value and the other stable value may depend on whether the circuit with hysteresis is inverting or non-inverting. In response to the input signal is between the high trigger point and the low trigger point, the output retains its current stable value. Such a circuit with hysteresis may be referred to as a bistable multivibrator. A Schmitt trigger is one example of a circuit with hysteresis. A circuit with hysteresis may be used for noise immunity and for waveform generators, such as an oscillator.
Automatic test equipment (ATE) is used to provide given input values to fabricated chips. A high-speed, complex ATE may be relatively expensive. A low-speed ATE consumes longer test times. Circuitry for built-in self-test (BIST) allows a design to test itself, but consumes on-die real estate for the test circuit and control logic. Tests for fabricated chip designs that include circuits with hysteresis verify whether the high and low thresholds are at acceptable values. Typically, two steps or tests are used. For a first test, an input voltage is swept from a logic high value to a given low threshold value at given increment steps, such as a fraction of a volt. The output values are recorded and checked. For a second test, the input voltage is swept from a logic low value to a given high threshold value at the given increment steps. The output values are again recorded and checked.
The above-described testing method for circuits with hysteresis consumes a lot of time. The cost of testing may not be great for characterization studies. However, for a large number of device packages for production, such as millions of packages, the cost is high. If the tests are executed only on a small number of packages at production time, then high-coverage testing is not achieved.
In view of the above, efficient methods and mechanisms for efficiently testing circuits with hysteresis are desired.